1. Field of the Invention
The current invention relates generally to alarms and signaling apparatuses for generating audio and visual indicators in response to an applied electrical signal, and more specifically, to digital electronic private automatic branch exchanges (EPABX's) with analog telephones which provide a "Message Waiting" feature.
2. Related Art
The current invention is a signal generator for a telephone system, such as a private branch exchange (PBX), which controls operation of audible and visual subscriber station instrument signaling devices.
In such systems a composite signal, comprising audible and visual control signals alternately spaced in time, selectively energizes the ringer or message waiting lamp in subscriber instruments. These audible and visual signals energize ringers and neon lamps respectively at subscriber stations of a PBX via a common bus, to which each station may be selectively connected, via an individual single wire path for each station. The current invention can be utilized with the circuit disclosed in U.S. Pat. No. 4,266,101 to Dunbar, which is hereby incorporated by reference.
A common problem on analog telephones which have the "Message Waiting" feature is "Bell Tap." Bell tap is transient energizing of the ringer which occurs when the message waiting signal is applied and removed.
A discussion of the basic principles of telephone ringers will aid in the understanding of the current invention. Many types and styles of ringers have been developed and placed in service. Some ringers will operate on alternating current only. Others will operate only on ringing current of a special frequency and are known as harmonic ringers. One of the most common is the polarized-biased type. An understanding of the theory of the polarized-biased type will give a working knowledge of practically any other type of ringer.
The polarized-biased ringer is a combination of a permanent magnet and an electromagnet. Ringer coils are wound on iron cores which are securely fastened to only the south pole of the permanent magnet. This combination is known as a ringer solenoid. The north pole of the permanent magnet is supported by the heel plate fastened to the signal case. However, its free end does not come in metallic contact with any of the other parts of the ringer.
By connecting the ringer coils securely to the heel plate, the permanent magnet establishes south poles at the free end of each coil. The armature, to which the clapper arm is attached, is hinged near the north pole of the permanent magnet. This causes the armature to have a north pole at each end while the center is the south pole. Therefore, it can be said the armature is a piece of soft iron with three poles; one at each end, and one in the center inducted by the action of the permanent magnet.
The biasing spring holds the armature nearer to one pole piece than the other. To ring the bells of the ringer it is necessary for the clapper arm to swing back and forth between the two bells. By alternately striking one and then the other in rapid succession a penetrating ringing tone is produced.
If current is caused to flow through the ringers in a certain direction the south pole of the right hand coil will be strengthened considerably and the strength of the left hand coil south pole will be weakened, neutralized, or reversed, depending upon the strength of the energizing current. The armature will be attracted to the right hand coil and repelled from the left hand coil. The effect of the biasing spring will be overcome and the clapper will move to the opposite side and strike the left bell.
When the current is removed from the coils, the magnetic relations in the ringer will return to the original state and the biasing spring will pull the armature back to its original position. In so doing, the clapper will strike the right bell and cause it to ring. If the current impulses energize the coils in rapid succession the bells will continue to ring.
When alternating current (AC) is supplied from the generator, the magnetizing effect of the alternating current will cause the armature to be attracted to first one coil and then the other, without the use of a biasing spring. However, a biasing spring would be necessary to place sufficient tension on the armature to keep the bell from tapping when the line is connected or disconnected from the central office battery. Unfortunately this technique does not always prevent bell tap.
When a pulsating direct current, either positive or negative, is applied to a circuit in which a condenser is in series with a ringer, the alternate charging and discharging of the condenser will cause the ringer to operate. In this instance the energizing effect of the current flowing from the condenser attracts the armature and the biasing spring brings the armature back to normal during the intervals of current flow.
When a common battery system is in use, the ringers on individual lines are connected directly across and the subscriber line is in series with a condenser. The ringing current can be supplied from the central office over the ring side of the line. The circuit is completed over the tip side of the line; hence this circuit is known as a metallic ringing circuit. The condenser opens the line to direct current. Therefore, the circuit for the line battery is not complete until the receiver is removed from the switch hook.
When the telephone is in operation, the ringers are connected across the line. However, their high impedance to the voice current prevents any interference with the operation of the receiver and transmitter. A more complete explanation can be found in Frank E. Lee, abc of the Telephone, Vol. 1, pp 40-41, incorporated herein by reference.
In addition to a ringer, some telephones possess a message waiting feature. One technique for implementing this message waiting feature is using a small neon bulb located on the face of the telephone near the rotary dial or dual tone multi-frequency (DTMF) keypad. The lamp flashes on and off to alert the user that a message is waiting for them with a receptionist/operator.
The neon bulb requires a small current and approximately 90 volts to ionize the neon gas in the bulb. This ionization causes the bulb to illuminate. The neon bulb is part of a circuit which allows the necessary voltage to be applied to the neon bulb. This circuit, in analog telephones, is wired in parallel with the audible alerting device, typically a ringer or bell.
The ringer is activated by applying alternating current to it. This current typically has a frequency of 20 hertz. The positive going edge of the Ring AC signal causes the clapper to be attracted to one of the bells in the telephone. Then, as the Ring AC signal becomes negative, the clapper is attracted to the opposite bell. If the attraction is strong enough bell tap occurs.
The Message Waiting signal and the RING signal are sent to the telephone on the same pair of lines, i.e., the RING and TIP lines. A ring relay determines which signal, if any, is sent to the telephone, i.e., either the RING signal, the Message Waiting signal or neither signal. A ring relay is a mechanical relay. Typically, the ring relay is a single pole, double throw relay. It has the capability of connecting a composite Ring AC/Message Waiting (RAC/MW) signal line to one of the subscriber's signal pair, i.e., the TIP or RING signal. The composite RAC/MW signal comprises AC signals and DC signal alternately spaced in time. The precise instant the relay is energized is contingent upon the desired result. If the PBX is trying to alert or "RING" the subscriber's telephone, the relay will be closed at the instant the Ring AC signal begins to appear on the RAC/MW signal line and opened at the instant the Ring AC signal terminates. The Message Waiting signal is unsuitable for ringing the bell because the Message Waiting signal is a direct current (DC) signal. The BELL requires an alternating current signal to sustain an audible ring longer than a single tap.
The requirement for the Message Waiting signal is that it be a direct current signal typically greater than 90 volts. However when the direct current signal is applied to turn on the lamp (rising edge) and when it is removed to turn off the lamp (falling edge), the signal looks to the bell like the beginning and ending of a Ring AC signal. Therefore, the rising edge will cause the clapper to be attracted to one of the bells. When the Message Waiting signal is removed, the collapsing field in the bell coil will cause the clapper to be attracted to the other bell.
U.S. Pat. No.4,533,792 to Binks et al., herein incorporated by reference in its entirety, describes a process for eliminating some causes of bell tap. Binks requires that the ring relay be closed at the zero crossover point in the Ring AC signal. The Binks patent does not disclose or suggest modifying the combined Ring AC/MW signal. Binks eases the problem of bell tap but does not eliminate it. Modifying the signals is a more reliable method because even when closing the relay at the zero crossover point the rise time of the Message Waiting signal is enough to cause bell tap in some situations. What is needed is a process that works more consistently and which works at the signal level by modifying the ring and Message Waiting signals.